Hundreds of millions of tons of wheat, corn, soybean, rice and other grains such as sorghum, sunflower seed, rapeseed, barley, oats and others, are dried in grain dryers. This is done for purposes of increasing allowable storage time, to maintain increased crop quality, and to reduce spoilage from fungi or molds, or insect damage.
While it is hard to generalize the moisture content of a typical harvest crop, grains are likely to have a moisture content of 17 to 30% by weight. If the grain is to be used for seed, that moisture content must be reduced to 9 to 12% to minimize the loss of germination during long term storage until the next planting season. The same is true for edible or commercial crops, especially if it is anticipated that the crop will be held in long term storage such as in a grain elevator.
The temperature and moisture content of the crop to be dried will generally be in relative equilibrium with the temperature and moisture content of the surrounding air in which it was harvested and/or is being stored. To reduce the crops moisture content that air has to be replaced with air at a reduced relative humidity. The most common way of achieving this goal is blowing heated air through and around the crop in a crop dryer. As a general, but rough, rule of thumb, for each 20° F. increase in air temperature results in a halving of the relative humidity of the heated air. If that heated air is then blown through and around the grain to be dried, it will extract moisture from the grain, thereby reducing the moisture content of the grain. There are a variety of various designs for crop dryers, but the vast majority rely upon the addition of heat to the drying air.
The typical fuel for heating the air is natural gas, if it is available for use at the storage location, or LP gas, typically propane, and of course, electricity may also be a fuel source for heating the drying air. Drying a crop is an energy intensive process, and inefficient since all that heated air is ultimately exhausted to atmosphere when moisture laden.
Another problem with using heated air for crop drying is the damage to the crop being dried if heated above certain temperatures. For example, while corn intended for use as cattle feed may be heated to about 180° F. without significant nutritional loss when used as cattle feed, but seed corn, to be used for next year's planting, cannot be heated to above 110° F. and preferably never above 105° F. If the corn crop is harvested in hot and humid conditions, the ambient temperature of the seed corn might be around 90° F. at the beginning of the drying process, and the upper limit temperature may be exceeded when heated air is introduced to dry the crop. These same problems of temperature limits are encountered with other types of grain crops too, for example rice, where the upper temperature limit may be 106° F., and if that limit is exceeded the rice kernels are degraded and the germs of the kernels may rupture.
There are low heat crop dryers found in the prior art, where ambient air is heated just a few degrees before being blown past the crop to be dried, but these systems often require days, if not weeks of continuous operation to adequately dry a crop, and may not always work adequately under high heat and humidity conditions.
The process of removing moisture from such commodities may be accomplished by closed-loop refrigeration methods. This involves forcing air over or through the subject product—i.e., the product from which moisture is to be removed—and then extracting moisture from the circulating air. The employed refrigeration systems typically include a heat pump, which comprises a condensing heat exchanger located outside of the crop dryer and an evaporator heat exchanger inside the crop dryer. Moisture from the circulating air adheres to the evaporator heat exchanger, thus lowering the relative humidity of the circulating air. After some time of operation, the moisture content of the subject product is reduced to a desired amount. But these systems cool a crop, and may not work efficiently if the crop to be dried is being stored in a cold climate, for example where the average ambient temperature of the crop in storage is 25° F. Frost damage can be encountered if a crop is dried using refrigeration under such conditions.
Another problem with prior art heat pump-based systems, however, is that the evaporator heat exchanger accumulates frost; and, at some point, the heat exchanger becomes so frosted that the evaporator coil no longer functions to remove moisture from the air. The typical solution to this frost issue is to cease operation of the heat pump in order to defrost the coil. This, of course, limits the effective operating time of the heat pump.
What is needed is a way of reducing the relative humidity of the drying air without significantly altering the temperature of the crop being dried either by the addition, or extraction, of heat from the drying air.